1. Field of the Invention
The present invention relates to an optical transceiver module whose submodule is accommodated in a metal package and which can intercept noise, having a highly reliable long life, and an optical communications system using the same.
2. Description of the Related Art
FIG. 1 shows a general structure of a single-fiber bi-directional optical communications system, in which a central station and a subscriber are connected together by one optical fiber. At the central station, optical signals having a wavelength of xcex2 (1.55 xcexcm) are generated by an LD 1 for providing to the subscriber. The optical signals are transmitted through a wavelength division multiplexer (WDM) 1 and is sent to an optical fiber OF. The optical signals transmitted through the optical fiber OF are subjected to wavelength division by a WDM 2 at the subscriber side, and the resulting signals enter and are received by a PD2 at the subscriber sides. These signals are a xe2x80x9cdownward signalsxe2x80x9d which are directed towards the subscribers from the central station and have a wavelength of xcex2.
On the other hand, at the subscriber side, signals for the central station are converted into optical signals having a wavelength of xcex1 (1.31 xcexcm) by an LD 2, and the converted signals are sent to the optical fiber OF through the WDM 2. The WDM1 demultiplexes the optical signals having a wavelength of xcex1 and the resulting signals are directed to a PD 1, by which the signals are converted to electrical signals. These signals are xe2x80x9cupward signalsxe2x80x9d directed towards the central station from the subscribers, and have a wavelength of xcex1. Since the wavelengths are different, it is possible to perform simultaneous bi-directional transmission. Obviously, ping-pong transmission can also be carried out. A WDM is provided at a branching portion of an optical transmission line (optical fiber, optical waveguide) and carries out wavelength selection.
An optical receiver module having a structure in which a light receiver device (photodiode (PD)), a lens, an optical fiber, etc., are disposed on an optical axis receives optical signals. In FIG. 1, the optical receiver modules are simply indicated as PD1 (central station side) and PD2 (subscriber side). An optical transmitting module having a structure in which a light emitting device (semiconductor laser, laser diode (LD)), a lens, and an optical fiber are disposed at the same optical axis generates optical signals.
The following types are actually and currently used as optical receiver modules. In one type, a PD chip is mounted to the top surface of a disk-shaped metal stem having lead pins protruding from the bottom surface, a cap holding a lens is secured to the stem, and a ferrule holding an end of an optical fiber is secured to the stem by a circular conical ferrule holder. In another type, a PD is attached to a metal stem, a cap having an opening is mounted to the stem, a circular cylindrical lens holder holding a lens is aligned with and secured at the outer side of the cap at the stem, a ferrule holding an end of an optical fiber is passed through a circular conical ferrule holder, and the ferrule holder is aligned with and secured on top of the lens holder.
Here, the lens is disposed at an axial line of the optical fiber, and the PD is disposed vertically on a line extending therefrom. Light exiting from the optical fiber propagates through space and is focused by the lens. The focused light enters the PD. Each type of optical receiver module is entirely enclosed by a metal package (stem, lens holder, and ferrule holder) by hermetic sealing, so it is highly airtight. Because moisture and oxygen do not enter the package, the PD chip, wiring, etc., do not deteriorate. Each type of optical receiver module is stringently enclosed by the metal package, so electromagnetic waves and light do not enter it, and therefore crosstalk is low. Each type of optical receiver module has been actually used for a long time, so it is highly reliable.
The optical transmitting module which is currently used is a metal-can-type package module which is accommodated in a similar metal package. In one type of optical transmitting module, a protrusion (called a xe2x80x9cpolexe2x80x9d) is formed at a disk-shaped metal stem, a semiconductor laser is vertically attached to a side surface of the pole, a monitor photodiode (monitor PD) is mounted to a stem surface that is disposed directly below the semiconductor laser, a circular cylindrical cap having a lens is secured on top of the photodiode, a ferrule holding an end of an optical fiber is supported by a ferrule holder, and the ferrule holder is aligned with and secured to the stem.
In another type of optical transmitting module, a cap only having an opening is mounted to a surface of a stem, a circular cylindrical lens holder holding a lens is mounted to the top surface of the stem disposed at the outer side of the cap, a ferrule holding an end of an optical fiber is supported by a ferrule holder, and the ferrule holder is aligned with and secured on top of the lens holder. These types of optical transmitting modules each accommodated in a metal package are enclosed by hermetic sealing, so they are highly reliable, have a long life, and prevent the occurrence of crosstalk because they do not allow the flow of electromagnetic waves and light to and from the metal package. A metal-can-type optical transmitting module and an optical receiver module are provided, being branched out by optical fibers from the associated WDM shown in FIG. 1. Therefore, there are an optical fiber which connects each metal-can-type optical transmitting module and its associated WDM, and an optical fiber which connects each optical receiver module and its associated WDM. The transmitting device and the receiver device of each module are independent devices, and are connected to their respective WDMs through optical fibers.
These modules having three-dimensional shapes and being entirely enclosed by metal packages have been used, are highly reliable, and will be subsequently used in the future.
Although, in the aforementioned description, an optical transmitting module and an optical receiver module are connected to a WDM by optical fibers, respectively, there are those integrally connected, such as disclosed in Japanese Patent No. 3167650, for example. A general structure thereof is shown in FIG. 2. A transmitting device 29 comprises an LD 32 and a monitor PD 33 mounted to a pole 31 inside a box metal package, and a condenser lens 34 provided in a front opening of the box package. A transmitting section is enclosed in the metal package by hermetic sealing and shielded from the outside. A receiver device 35 comprises a box metal package 36, a submount 37, a circular cylindrical cap 38, a condenser lens 39, and a PD chip 40.
These modules are independent modules, but are integrally formed by an integration metallic housing 41. The transmitting device 29 is welded and secured to an opening on an extension line from an axial line of the integration housing 41. The receiver device 35 is welded to the integration housing 41 so as to face a side opening of the integration housing 41. Light emitted from the LD 32 of the transmitting device 29 is focused by the condenser lens 34, and the focused light is transmitted through a WDM 42 and enters an optical fiber 43. The light that has propagated through the optical fiber 43 is reflected by the WDM 42, and the reflected light is transmitted through a condenser lens 39 from the side opening, and is incident upon and received by the PD chip 40.
In this manner, the independent transmitting device (optical transmitting module) and receiver device (optical receiver module), which are enclosed in metal packages, are integrally formed by the housing. However, these modules are still independent, separate modules. Therefore, these modules are nearly the same as a transmitting device and a receiver device that are connected to a WDM by optical fibers.
However, the extent to how small the optical transceiver module which have a metal package and a three-dimensional structure having an optical axis of an optical fiber in a direction of the normal to the stem can be made is limited because the metal packages have specific sizes. In addition, the extent of cost reduction is also limited. Even the special integrated optical transceiver module in FIG. 2 cannot reduce the volume, so this optical transceiver module is large in size and expensive.
A promising optical transmitting module and optical receiver module which are small and low cost are a surface-mount-type optical transmitting module and optical receiver module. These modules have a V-groove, an optical waveguide, etc. formed at an Si platform. A PD and an LD are mounted to the trailing ends thereof, and an optical fiber and the optical waveguide are abutted against them. The axial line of the optical fiber becomes parallel to a surface of the platform. Since the parts are mounted onto the Si platform having a V-groove or a mark, the parts can be easily positioned and mounting precision can be achieved even if time-consuming aligning is not carried out. A method for mounting parts which does not include an aligning operation is sometimes called xe2x80x9cpassive alignment.xe2x80x9d Since the optical fiber and the PD and LD are disposed close to each other, high optical coupling efficiency can be achieved.
In other words, a lens is not required. Since a lens is not used and the optical fiber and the PD and LD are disposed close to each other, the modules can be reduced in size. A plastic package in which the Si platform and a lead frame are plastic-molded together is used instead of a metal package, and therefore the module can be made light and small and reduced in cost.
In addition, a module can be designed to carry out both transmission and reception by forming an optical waveguide on the Si platform, with the LD being disposed at an end of the optical waveguide, a WDM being obliquely placed in the middle thereof, and the PD being disposed obliquely upward from the WDM. This type of module is called an optical transceiver (LD/PD) module.
As mentioned above, a surface-mount-type optical receiver module and a surface-mount-type optical transmitting module have many advantages. However, for applications in harsh environments, there is a persistent desire to enclose the optical elements in metal packages by hermetic sealing because hermetic sealing makes the metal packages highly air-tight.
An optical receiver module having only an optical receiver device (PD) mounted in a metal-can-type package is available and is currently in use. An optical transmitting module having only an LD or an LD and a monitor PD mounted in a metal package is often used with this type of optical receiver module.
Accordingly, it is an object of the present invention to provide a metal-package-type optical transceiver module which makes use of features of a surface-mount type and which makes it possible to perform hermetic sealing.
According to one aspect of the present invention, an optical transceiver module comprises: a submodule comprising a platform, a Y-branching optical waveguide disposed thereon, a light receiver device, and a light emitting device, the Y-branching optical waveguide having wavelength selectivity, with its leading end exposed at the front surface of the platform and two trailing ends c and d thereof being branched at a branching portion, for separating wavelengths into a first wavelength xcex1 and a second wavelength xcex2, the light receiver device for receiving the second wavelength xcex2 being disposed on the platform so as to oppose and to be close to a first trailing end of the optical waveguide, the light emitting device for generating the first wavelength being disposed on the platform so as to oppose and to be close to a second trailing end of the optical waveguide; a plurality of lead pins extending in an axial direction; a disk-shaped metal stem having an upwardly protruding pole, the stem holding the submodule at a surface of the pole; an optical fiber whose end is secured relative to the stem; a lens for optically coupling the optical fiber and the leading end of the Y-branching optical waveguide; a cap secured on the stem and holding the lens; and a supporting member secured to the stem and holding the optical fiber.
An optical transceiver module according to another aspect of the present invention comprises: a submodule comprising a platform, a waveguide disposed on the platform with the leading end xe2x80x9cexe2x80x9d thereof being exposed at the front surface of the platform, a groove disposed in the middle of the waveguide, a multiple layer filter inserted in the groove and having wavelength selectivity for transmitting a first wavelength xcex1 and reflecting a second wavelength xcex2, a light receiver device, which is disposed above the platform so as to be close to and to oppose the multiple layer filter and receives the second wavelength xcex2, and a light emitting device, which is disposed on the platform so as to be close to and to oppose the trailing end xe2x80x9cfxe2x80x9d of the optical waveguide and generates the first wavelength xcex1; a plurality of lead pins extending in an axial direction; a disk-shaped metal stem having an upwardly protruding pole for holding the submodule at a surface thereof; an optical fiber whose end is secured relative to the stem; a lens for optically coupling the optical fiber and the leading end xe2x80x9cexe2x80x9d of the optical waveguide; a cap secured to the stem and holding the lens; and a supporting member secured to the stem and holding the optical fiber.
The optical transceiver module of the present invention provides the following advantages.
In contrast with a conventional module, which is either a transmitting device or a receiver device because a metal package can only accommodate either a light receiver device or a light emitting device, in the present invention, both of these elements can be accommodated in a metal package such that the module is an optical transceiver module, since a submodule comprising a Y-branching optical transmission line provided on the Si platform and a light emitting device (LD) and a light receiver device (PD) disposed at the trailing ends of the transmission line can be accommodated in the metal package.
Thus, the module is an optical transceiver module, and nevertheless it can exhibit the same long-lasting reliability as a conventional optical receiver module or optical transmitting module that is accommodated in a metal package, since the package is durable and has a good record of performance. An axially symmetrical submodule is used, which facilitates alignment with the optical fiber and the lens. The optical transceiver module has fewer parts, is smaller, and is lower in cost compared to the optical receiver module and optical transmitting module that are separately provided in metal packages. Since the module is down-sized, it can be advantageously used as an optical transceiver module at a station side.